Wikipedia:Reference desk/Archives/Science/2023 June 10

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June 10

Curl B fields and electric plates.

Let's say we have 2 electric plates hooked up to an AC power supply. Let's say we can turn on the power in such a manner that we get a changing E field between the plates but no electrons jumping from plate to plate (not sure if this is possible but we'll make it a thought experiment in any case).

Does the changing E field between the plates produce a curl B field? I point out here that there is no moving charges here, just a changing E field. In reality, of course, it would be very hard to determine the difference between any curl B fields produced by the rest of the circuitry, but let's see what people have to say anyway or if someone can point me to a known experiment of this type. Byron Forbes (talk) 06:43, 10 June 2023 (UTC)

The curl of a magnetic field at any given point is equal to the current density vector at that point. No current, no curl.  --Lambiam 06:58, 10 June 2023 (UTC)

I'm a bit puzzled why you would say that. The vacuum Maxwell equation without current density still reads $\nabla \times \mathbf {B} =\mu _{0}\varepsilon _{0}{\frac {\partial \mathbf {E} }{\partial t}}$; this cannot be solved in isolation, of course, since Maxwell's equations are coupled and need to be solved together. In the case of the parallel-plate capacitor, it will emit a pulse of electromagnetic radiation while being charged. The detailed shape and frequency spectrum of that will depend on the details of how the capacitor is charged. --Wrongfilter (talk) 14:53, 10 June 2023 (UTC)

You are right. Many sources just state a "differential form" of Ampère's law that ignores the electric displacement field, like here equation 2.43 for $\mathbf {H} ,$ and I copied that without looking further. The fluxion of that field should be added.  --Lambiam 17:18, 10 June 2023 (UTC)

I disagree.

I think curl B fields are produced only in the presence of charged particles and not by a raw E field. Maxwell makes a clear distinction here that this is a displacement current and also alludes to electric dipoles.

Current in a wire represents rising and falling E dipoles due to electron motion relative to nuclei.

Also, to consider a string of electrons moved past a point, such a situation also creates a virtual dipole rising and falling as the field at that point changes. Indeed, 1 electron on its own can do likewise.

I was almost going to mention that the explanation on the "Maxwell's Equations" page was "daring" and "bold" since it refers to mere "electric fields" on numerous occasions. :)

Having said this, it is basically impossible, I presume, to have a changing E field without moving charged particles being involved. I believe they are inseparable things. Byron Forbes (talk) 05:40, 11 June 2023 (UTC)

.................and, come to think of it, I see what's happened here.

For a photon to "work" far from charged particles, we must have a raw E field generating a raw B field, etc, etc, of course. We wouldn't want any evil aether particles around and required, now would we? :) Byron Forbes (talk) 05:51, 11 June 2023 (UTC)

Perhaps you should take your crusade against the developments in physics since 1850 to a different forum.  --Lambiam 10:30, 11 June 2023 (UTC)

LOL - that hit a nerve, didn't it?

And here is where I got the reminder, right here on Wikipedia on this page https://en.wikipedia.org/wiki/Maxwell%27s_equations, under the heading Ampère's law with Maxwell's addition, 2nd paragraph -

Maxwell's addition to Ampère's law is important because the laws of Ampère and Gauss must otherwise be adjusted for static fields.[clarification needed] As a consequence, it predicts that a rotating magnetic field occurs with a changing electric field. A further consequence is the existence of self-sustaining electromagnetic waves which travel through empty space.

Good old Relativity people assuming theory to prove theory and adjusting everything to line up with their theory. Changing fields mean there are local, moving charged particles - this not being the case is unlikely in the extreme!

Not that I suggest any famous scientist ought to be treated as a god, but I think it is clear here that Maxwell specifically did not mean a mere, raw E field! Byron Forbes (talk) 13:33, 11 June 2023 (UTC)

Thank you.

And, FWIW, I agree. Byron Forbes (talk) 10:21, 10 June 2023 (UTC)

See displacement current (if you have not already found that article), which directly addresses the question of current in capacitors. catslash (talk) 18:17, 10 June 2023 (UTC)

I wonder how they make the distinction between all the stuff the electrons are doing everywhere about the plates and the E field itself. Do they really believe that the curl B is only directly perpendicular out of the wire? There will be a curl B field "between the plates" as a consequence of all the local electron movement anyway! There will even be perpendicular fields due to electron flow along the plates!

This is all very presumptuous and convenient for Relativity. Byron Forbes (talk) 17:24, 16 June 2023 (UTC)

Maximum depths of a theoretical ocean

How deep could a theoretical water ocean be before it starts forming ice under the pressure of the water above? Also, how would that change for arbitrary values of temperature/gravity/salinity? Is it even reasonable to assume a constant temperature/salinity (or gravity even?) for the entire depth? I've no clue how to even begin calculating this from an assumption of an infinite plane of ocean, and googling hasn't been helpful either. I'd appreciate any pointers. ― Synpath 17:38, 10 June 2023 (UTC)

OK, skipping the chatbot response... it sounds like what you're looking for is some sort of equation for the freezing point of water at a given pressure and salinity; an equation for pressure at a given depth; and possibly equations for the interactions of salinity and pressure, and salinity and depth? I don't have that information, nor do I know where to find it, but maybe someone else does. -- Avocado (talk) 16:10, 11 June 2023 (UTC)

I ended up calculating a ~100 km depth from a 1 m² column of water and seeing it matched an answer below I suppose its close enough to correct. The notion of Brine rejection was useful in that it lead me to these two papers: [1] and [2] where the first has some nice visuals of it for high pressure ice phases (Fig. 1 and Fig. 3) and the second has a phase diagram based on water LiCl solutions (Fig. 1) where the phase boundary between liquid and Ice VI is roughly in the same order of magnitude as for pure water. So, while salinity would increase the final depth it won't be doubling it or anything assuming the sodium hydration shell radius doesn't change things much. ― Synpath 18:37, 11 June 2023 (UTC)

Warning: This statement by Bing chatbot may contain unverified speculation.
The following discussion has been closed. Please do not modify it.
One of the most fascinating phenomena in the deep ocean is the formation of ice crystals under high pressure. The water at the bottom of the ocean is very cold, close to freezing point, but it does not freeze because of the dissolved salts and minerals. However, when the pressure increases due to the weight of the water above, some of these dissolved substances are squeezed out of the water molecules, making them more pure and less dense. This allows them to form ice crystals, called ice VII, that have a different structure than normal ice. These crystals can float up to the surface, creating a layer of slushy ice under the water.

Do not rely on the statement above. Philvoids (talk) 19:18, 10 June 2023 (UTC)

Take into account that from a certain depth, the mantle heat will melt water under higher pressure. The water phase diagram you already used will help to determine if water is liquid at a combination of pressure and temperature.

Zarnivop (talk) 20:27, 10 June 2023 (UTC)

In Earth gravity, the pressure increases about 10⁴ Pa/m. At a pressure of about 10⁹ Pa ice VI will begin to form. That means that the ocean can be about 100 km deep. With more gravity, it will be shallower; with higher temperature, it will be deeper; with higher salinity, it will be deeper too. But it will be on that order of magnitude. BTW, amazing what kind of rubbish that chatbot can make up. I at least know that I know nothing. PiusImpavidus (talk) 23:33, 10 June 2023 (UTC)

Impossible to answer for a real ocean by calculation alone. For water it depends on the temperature and salinity at a given depth. At 20 deg C and 0% salinity the pressure would have to be 10000 atmospheres, so 100 km down, roughly. If colder, less, if salty, more. Greglocock (talk) 23:59, 10 June 2023 (UTC)

I am sceptical of the chatbot's notion that water pressure can "squeeze....dissolved substances out of water molecules". This seems applicable to Desalination of seawater but the only technologies known for this are Distillation or Reverse osmosis through a Semipermeable membrane. Philvoids (talk) 00:45, 11 June 2023 (UTC)

To the notion of dissolved substances being somehow squeezed out... where would they go? ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:47, 11 June 2023 (UTC)

The unstated implication is that a gradient of gradually rising concentration would arise in the water column, so they would effectively be gradually transferred upwards, but I agree that the entire premise is at best questionable and unverified, although the chatbot (which merely strings together words associated in its corpus within grammatical constraints) may have got them from somewhere. {The poster formerly known as 87.81.230.195} 90.221.195.5 (talk) 07:47, 11 June 2023 (UTC)

Hmmm... So, given the premise that the salt would rise, how would that salt have gotten to the bottom of the ocean in the first place? ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:15, 11 June 2023 (UTC)

With freezing the water crystalizes and rejected salt creates a denser brine that is forced out of pockets called brine cells. The brine sinks and the purer ice floats. See Brine rejection. Modocc (talk) 08:19, 11 June 2023 (UTC)

See, this is the problem I raised on the Talk page. We're earnestly discussing the possible correctness of a vaguely plausible-sounding passage assembled by a chatbot because it didn't cite a source (and if it had it might have cited a fake one). To avoid this sort of thing becoming a huge time-sink, I propose we henceforth ignore all chatbot-generated contributions to Ref desk answer threads. {The poster formerly known as 87.81.230.195} 90.221.195.5 (talk) 14:22, 11 June 2023 (UTC)

You may have ignored the warning. There is also an obvious red flag in the chatbot's Weasel worded introduction "One of the most fascinating phenomena..." But supposing you saw the chatbot statement without its first sentence, how do you propose handling it? Ernest fact-checking is urgently called for as our best human reaction now that chatbots are flourishing and may even co-opt the chant WE'RE QUEER, WE'RE HERE, GET USED TO IT! Philvoids (talk) 16:39, 11 June 2023 (UTC)

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